Method of coating an electrical unit with ceramic material



1970 L. H. BERKELHAMER 3,489,828

METHOD OF COATING AN ELECTRICAL UNIT WITH CERAMIC MATERIAL Original Filed Feb. 9, 1962 REF/94c rwzy D/GRED/EN r5 MELT/NG- a dr- FI/RIVACE Wars/g Wow-620.55

CERAMIC HATER/41,6

United States Patent 3,489,828 METHOD OF COATING AN ELECTRICAL UNIT WITH CERAMIC MATERIAL Louis H. Berkelhamer, Glencoe, Ill., assignor to Consolidated Electronics Industries Corp., Ohmite Manufacturing Company Division, Skokie, 11]., a corporation of Delaware Continuation of applications Ser. No. 172,276, Feb. 9,

1962, and Ser. No. 450,703, Apr. 26, 1965. This application Apr. 14, 1967, Ser. No. 631,095

Int. Cl. C041) 39/00; B28b 1/16; B32h /04 US. Cl. 26461 6 Claims ABSTRACT OF THE DISCLOSURE A method of forming a composite electrical unit comprising the steps of mixing vitreous ceramic material and non-vitreous ceramic material with water to form a plastic mixture, forming a preform of the plastic mixture to fit as an envelope upon the composite electrical unit, compressing the preform against the surface of the electrical component to form an envelope enclosing the electrical unit, heating the assembled composite electrical unit to a temperature to cause an outer shell having a nonvitreous material bonded together by a glassy matrix.

The present application is a continuation of my copending applications Ser. No. 172,276, filed Feb. 6, 1962, and application Ser. No. 450,703, filed Apr. 26, 1965, both now abandoned.

This invention relates generally to small electrical components, especially to small electrical components that incorporate a vitreous enamel coating, and more particularly to methods'of coating an electrical unit with ceramic material.

One construction which is commonly employed in producing numerous types of small electrical component involves wrapping or otherwise situating an electrical operating element on the surface of a relatively rigid core whereupon an insulating coating is applied as a relatively watery slip over the electrical element and over the core to complete the unit. When the core and the insulating coating are selected to be fabricated from ceramic materials, the unit possesses the desired attribute of being non-flammable. Moreover, when the ceramic coating is vitrified in situ, the resultant intimate contact between the coating and the electrical element promotes dissipation of the heat generated during use of the unit.

In acordance with conventional procedures, vitreous enamel coatings are produced by applying to electrical units a liquid slip comprising finely ground glassy ceramic material, coloring ingredients, fluxing agents and sufficient water to develop a syrupy mixture. This slip is applied to the electrical units by either the dip or the spray process whereupon the coated electrical units are fired to fuse the glass frit. During firing of the units, the vitreous enamel material reaches a highly fluid state; and the electrical units are commonly rotated in the firing furnace in order to reduce run-off of the enamel. Furthermore, the electrical operating elements may move relative to the core due to the flow of the enamel during firing; and there is the problem of the coating burning through at thin spots. According to the conventional procedures for producing a ceramic coated electrical component, there is a need for much careful inspection, reworking and cleaning; and in addition, the fluidity of the enamel during the firing step prevents controlling of the gross shape of the unit with any degree of precision.

Therefore, a general object of the present invention is to provide a new and improved method of coating an electrical component with molded ceramic material.

3,489,828 Patented Jan. 13, 1970 Another object of the invention is to provide a method of covering an electrical component with a ceramic coating having a precisely controlled overall shape.

Still another object of the invention is to provide a method of surrounding an electrical component with a ceramic envelope, or jacket, which intimately contacts the encompassed elements and which possesses a substantially uniform thickness.

Additional objects and features of the invention pertain to the particular materials and methods whereby the above objects are attained.

Whereas the procedures of the prior art utilize a fluid enamel slip and embrace firing of the slip to a molten, fluid state, the method of the present invention contemplates use of a plastic ceramic mixture molded into a preformed jacket which is fixed in position about the electrical unit and compressed into intimate contact therewith for good heat transfer and size and shape control. Only a portion of the ceramic materials contained in the jacket is then sintered, the remainder of the ceramic materials remaining solid at all times to give the jacket suflicient integrity to prevent its changing its molded shape during the sintering.

In order that the principle of the invention may be readily understood, a single embodiment thereof applied to the coating of an axial lead resistor, but to which the application is not to be restricted, is shown in the accompanying drawing wherein:

FIG. 1 is a schematic diagram illustrating the steps to be taken in producing a plastic mixture in compliance with the present invention;

FIG. 2 is a perspective view of a green ceramic preform made from the plastic ceramic material resulting from the procedure of FIG. 1;

FIG. 3 shows the preform of FIG. 2 being assembled over an axial lead resistor;

FIG. 4 is a perspective view of the resistor of FIG. 3 with the finished ceramic coating disposed thereover; and

FIG. 5 is an enlarged view taken along the line 55 of FIG. 4.

The first major step to be taken in the practice of the present invention resides in producing a glassy ceramic material; and with specific reference to FIG. 1, suitable raw materials are charged in selected proportion into a melting furnace 10. These raw materials comprise silica; fluxing agents such as borax, soda ash and fluorspar; refractory ingredients such as feldspar and clay; and miscellaneous ingredients including suitable coloring materials and, if desired, opacifiers such as tin oxide, antimony oxide and zirconium oxide. The raw materials are held in the melting furnace from one to three hours at a suitable temperature such as for example 2500 F.; and after the batch has been uniformly melted, it is allowed to pour from the furnace 10 into a tank 12 containing a quantity of cold water. Contact with the cold water in the quenching tank shatters the melt into numerous, friable particles commonly referred to as frit. The substantially completely vitrified ceramic material which is taken from tank 12 is subsequently ground in a ball mill 14.

The second major step to be taken in the practice of the present invention involves blending the particulate, glassy ceramic material with non-glassy ceramic material having a higher fusion temperature than the glassy ceramic material and small quantities of water to produce a plastic mixture; and while the non-glassy ceramic material could be blended with the glass frit in the ball mill 14, it h s proved more convenient to store the produce from ball m ll 14 for subsequent blending with the nonglassy ceramic material and water in a ball mill 16. The non-glassy ceramic maerials which are suitable for use in the invention include clay minerals and the refractory oxides commonly utilized in the ceramic art. The desired plasticity in the mixture is obtained by regulating the water content or by incorporating a plasticizing, ceramic material; and when hot molding of the plastic mixture is contemplated, the plasticizer material is advantageously selected to be of a type which flows upon the application of heat whereby to facilitate molding of the mixture. The plastic mixture of water, glassy and non-glassy ceramic materials which is withdrawn from the ball mill 16 is shown collected in a pile 18.

The third major step to be taken in the practice of the instant method resides in applying a measured quantity of the plastic ceramic mixture to the surface of an electrical unit; and while the plastic mixture can be molded directly about an electrical unit, it is also contemplated to fashion the plastic mixture into a suitably shaped preform element 20 illustrated in FIG. 2. Depending upon the shape which is required, the preform 20 may be either molded or extruded and cut to size.

In the illustrated embodiment, the electrical unit which is to be coated with ceramic material is fashioned as an axial lead resistor indicated generally by the numeral 22 in FIG. 3. It is to be recognized, however, that the electrical unit could as well comprise a rheostat, a tap switch or some other electrical unit for the purposes of the invention.

The axial lead resistor 22 specifically includes a rigid, ceramic core 24; and in one particular application, the core 24 has been fashioned from the mineral substance known as steatite. Core 24 takes the form of a cylinder, and conductive metal caps 26 are assembled to the ends of core 24 in a friction fit. In accordance with conventional practice, lead wires 28 are mechanically and electrically connected to the end caps 26 to extend along the axis of the cylindrical core 24. An electrical operating element 30 takes the form of a metallic wire element which is helically wound on the core 24 between the end caps 26.

The preform element 20 is fashioned as a tubular member having an internal diameter sufficiently large to permit the reception of electrical unit 22. Such assembly of the preform element 20 and the electrical unit 22 is indicated in FIG. 3. After the preform element and the electrical unit have been assembled, pressure is applied to the resultant assembly to mold the preform element into intimate contact with the electrical unit, compacting the green ceramic plastic mass to a slight degree and forming portions of the mass over the end 26 to encompass the lead wires 28 emanating therefrom.

Upon forming the element 20 into the desired shape about the remainder of the electrical unit and in compliance with the fourth major step of the instant method, heat is applied to the assembly to fuse the particles of glassy ceramic material and to transform the green ceramic preform element 20 into a vitrified body 32 which defines a monolithic envelope surrounding the electrical unit 22 as is shown in FIG. 4.

In particular accord with the present invention, the entire body of ceramic material is not raised to a molten state. Thus, the original size and shape of the molded body is preserved, ensuring uniform electrical insulation of the unit, maintaining the originally established position of the electrical element, facilitating rapid dissipation of heat generated by the unit in use, and permitting automation of assembly of the completed resistor with other components into an electrical device or appliance. The original geometry of the ceramic body, which is established upon molding of the preform element 20, is maintained either by controlling the amount of heat applied or by controlling the ration of glassy to non-glassy ceramic material in the mixture. When the former approach is utilized, sufiicient heat is applied to raise the temperature of the glassy ceramic material to incipient melting whereby to fuse contacting inner faces, sufiicient heat to produce a fiowable mass being avoided. When the latter ap roach is employ a rela ive y h gh prop r on o non-glassy ceramic material is employed whereby to form a comparatively immobile, skeletal body which preserves the molded shape during sintering of the glassy ceramic material.

With reference to FIG. .5, the completed coating 32 is seen to comprise particles of non-glassy ceramic material 34 bound up in a matrix of fused glassy ceramic material 36.

After the glassy ceramic materials have been fused together, the completed unit is allowed to cool prior to packaging and shipment for use.

The specific example herein shown and described should be considered as illustrative only. Various changes may occur to those skilled in the art; and these changes are to be understood as forming a part of this invention insofar as they fall within the true spirit and scope of the appended claims.

-I claim:

1. The method of forming a composite electrical unit including a ceramic core component supporting an electrical component, and a molded vitreous ceramic envelope therefor; said method comprising the steps of: mixing a ground frit comprising a granulated non-vitreous ceramic material having a predetermined fusion temperature and a granulated vitreous material having a lower fusion temperature with sufiicient water to form a plastic ceramic material mixture having particles of the nonvitreous material dispersed therethrough, forming a preform of said plastic ceramic material mixture to fit upon the composite electrical unit; placing the preform upon the composite electrical unit; compressing the preform to a predetermined shape against the outer surfaces of the ceramic core component in intimate contact therewith to form an envelope for the ceramic core component and the electrical component supported thereby; and heating the assembled composite electrical unit and the envelope above said lower fusion temperature but below said predetermined fusion temperature for pyrochemically bonding the envelope to the composite electrical unit with the electrical component substantially completely enclosed thereby to create an outer shell having the nonvitreous material bonded together by a glassy matrix.

2. The method of claim 1 including the addition of a plasticizer with the water.

3. The method of claim 1 wherein the core component is substantially cylindrical with the electrical component wound thereon, and including the formation of the preform to tubular shape to fit around the electrical unit.

4. The method of claim '3 in which portions of the preform are formed over the ends of the core component with electric lead wires projecting therethrough from the electrical component.

5. The method of completing the formation of a composite electrical unit having a core component supporting anelectrical component by molding a vitreous ceramic envelope to enclose the composite electrical unit; said method comprising the steps of: combining a vitreous ceramic material with at least one non-vitreous ceramic material having a higher fusion temperature than the vitreous ceramic material and sufiicient water to make a vitrifiiable plastic mixture; forming a preform of the plastic mixture and associating the same with the composite electrical unit as an envelope; compressing the preform upon the surfaces of the core component to form an envelope in intimate contact therewith and enclosing the electrical component; and heating the resultant assembly to a temperature sufliciently high to cause incipient melting of the vitreous ceramic material but insufficiently high to cause fusion of the non-vitreous ceramic material but insufliciently high to cause fusion of the non-vitreous ceramic material.

6. The method of claim 5 wherein the non-vitreous ceramic material is in the plastic mixture in a relatively high proportion to define a comparatively im obile.

5 6 skeletal body upon fusion of the vitreous ceramic ma- 3,425,121 2/1969 Ostrander 264-61 terial. 3,436,451 4/1969 Wasser 264-61 References Cited UNITED STATES PATENTS DONALD J. ARNOLD, Primary Examiner 1,929,425 10/1933 Hermann 264-61 5 2,718,577 9/1955 Sherk 264-61 CL 3,144,318 8/1964 Bruen et a1. 264-61 264-62, 241, 256, 272 

